The present invention relates to a process for coating fibers with a thin, surface-protective, adhesion-promoting layer of silicon by means of sputtering.
At higher temperatures carbon fibers are not generally stable in contact with metals that form carbides, such as aluminum, so if they are intended to reinforce such metals they must be provided with a protective diffusion barrier before being embedded in the metal matrix. Protective layers are also necessary for fibers that are exposed to oxidizing media, especially at higher temperatures. Silicon carbide provides a suitable protective surface layer, being chemically stable to metals, resistant to abrasion, light, and resistant to oxidation and having low thermal expansivity.
It is known that carbon fibers are treated by various methods of forming surface-active groups before being embedded as reinforcement in matrices of synthetic resins; these groups, which improve adhesion between the fiber and the polymer matrix, can for instance be formed by superficial thermal, wet, or electrochemical oxidation. Besides methods of superficial oxidation, processes for forming polymer coatings are also known; they comprise radiation-induced graft reactions and electropolymerization at the surface of the fiber, which can be carried out anodically or cathodically. The polymer coating promotes adhesion between fiber and matrix.
Many processes for coating carbon fibers are known, but coating from the gas phase by the process known as chemical vapor deposition (CVD) is the one used most often.
These processes have the disadvantage that the temperature of the substrate has to be between 800.degree. C. and 1200.degree. C. if it is to be coated with a layer of silicon carbide, and are hence suitable solely for coating carbon or silicon carbide fibers. Because of the necessity of using high temperatures there is only a limited possibility of influencing the morphology and structure of the coating, and hence its chemical, physical, and mechanical properties and its adhesion to the surface of the fiber.
The use of vaporization processes for coating fibers with silicon carbide (which necessarily involve a chemical reaction: silicon reacts with an atmosphere of hydrocarbon) requires that the individual filaments in a bundle of fibers should be first spread out mechanically, since the stream of particles emitted by the vapor source is only slightly dispersed geometrically. In addition, the reactive components must be activated by high substrate temperatures or by the use of a plasma.
German Patent No. C 32 49 624 describes the manufacture of fibers with a superconducting layer of a niobium compound by the reactive direct-current sputtering of niobium. Here too the bundle of fibers had to be spread out mechanically, which led to the rupture of fibers.
Japanese Patent No. A 119 222/85 describes carbon fibers with a refractory coating, silicon carbide for instance. The coating is preferably applied by chemical vapor deposition (CVD); other methods, such as sputtering, are mentioned without any details.
The object of the present invention was to provide a technically simple process for coating reinforcing fibers with a surface-protective, adhesion-promoting layer of silicon, this process allowing many individual fibers in a bundle of fibers to be homogeneously coated at the same time.
We have found that this object is achieved if the coating is applied by radio-frequency sputtering.